A Review of the Mongolian Cretaceous Dinosaur Saurornithoides (Troodontidae: Theropoda) Author Norell, Mark A. Author Makovicky, Peter J. Author Bever, Gabe S. Author Balanoff, Amy M. Author Clark, James M. Author Barsbold, Rinchen Author Rowe, Timothy text American Museum Novitates 2009 2009-06-25 3654 1 64 http://www.bioone.org/doi/abs/10.1206/648.1 journal article 10.1206/648.1 0003-0082 5357410 Saurornithoides mongoliensis Osborn, 1924 HOLOTYPE : AMNH FR 6516 , a weathered skull with articulated mandible and associated partial postcranial skeleton. The outer surfaces of the skull and skeleton are highly weathered, and the postcranial skeleton is fragmentary. The skull is missing the dorsal and lateral walls of the braincase and the occiput dorsal to the foramen magnum, and most of the frontals were eroded away (fig. 3). The lower temporal and posttemporal bars are missing on both sides except for a suborbital section of the jugal on the right. The head of the right quadrate is preserved but not the left, and the upper temporal bars are missing on both sides. The illustration of the right side of the skull by Osborn (1924 ; fig. 4) combines some aspects of the left: the interfenestral bar is preserved only on the left side, but is illustrated on the right side. Both dentaries are preserved but are eroded ventrally. Only the anterior parts of the postdentary bones are preserved on both sides. The preserved postcranial elements include four dorsal vertebrae, six sacral vertebrae, the first four caudal vertebrae, a partial left ilium, parts of both pubes, parts of both ischia, the proximal part of the left femur, the impression of the left tibia, and the distal end of the left pes. Most of these bones were preserved in a concretion, with which a few isolated bones were associated. Many of the elements were removed from the concretion after Osborn’s description. A cast of the pelvic region in the concretion was made before the specimens were removed and is figured here (fig. 5). An isolated series of three caudals originally was referred to the holotype (fig. 6). These caudals are short, have rounded centra, and bear tall, rodlike neural spines. We reinterpret these caudal elements as being midcaudal vertebrae of a protoceratopsian, which probably were surface collected as a nodule near the Saurornithoides mongoliensis holotype and mistakenly referred to it. These vertebrae are now catalogued as AMNH FR 30613 . TYPE LOCALITY : Djadokhta Formation at Bayan Zag, Omnogov Aimag , Mongolia (figs. 2, 7). Unfortunately, the precise locality at the Flaming Cliffs where the specimen was collected was not recorded. The page from Walter Granger’s field book cataloguing the specimen is reproduced in figure 1. AGE : The Djadokhta Formation is considered to be Late Campanian based primarily upon the vertebrate fauna ( Loope et al., 1998 ; Dingus et al., 2008 ). DIAGNOSIS: Same as for Saurornithoides . REFERRED SPECIMENS: Two incomplete troodontid specimens from the Djadokhta Formation have been referred to Saurornithoides mongoliensis . Currie and Peng (1993) referred a specimen, IVPP V10597 , from Bayan Mandahu, China , to Saurornithoides mongoliensis . The specimen comprises a left hind limb including the femur, a partial tibia and fibula, tarsals, and an articulated foot. However, the hind limbs of the Saurornithoides mongoliensis and Zanabazar junior holotypes lack features diagnostic at the species or genus level, so we consider IVPP V10597 to be referable only to Troodontidae . Norell and Hwang (2004) provisionally referred a fragmentary specimen from Ukhaa Tolgod (IGM 100/1083) to Saurornithoides mongoliensis , but it, too, lacks features diagnostic of this species. The specimen includes a maxillary fragment, quadrate, six vertebrae, and a few bones from the feet. The pattern of tooth placement in the maxilla is similar to that of Saurornithoides mongoliensis rather than Byronosaurus jaffei ( Makovicky et al., 2003 ) , the only other Fig. 3 . Continued. troodontid described from this formation. It is likely that this specimen and the Chinese specimen are Saurornithoides mongoliensis based on their stratigraphic occurrences, but the lack of definitive morphological evidence negates a generic level taxonomic assignment.